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Thread: drawing on a QPixmap or QImage

  1. #1
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    Default drawing on a QPixmap or QImage

    Can you draw on a QPixmap/ QImage?

    I tried the following but it doesn't work:

    Qt Code:
    1. QPixmap pixmap;
    2. if ( pixmap.load("myImage.png") ) // load and draw image
    3. p->drawPixmap( 100, 85, pixmap );
    4. QPainter pixPaint(pixmap); // Error: Can't convert from QPixmap to const QPaintDevice . But doesn't QPixmap inherit QPaintDevice???
    5. pixPaint.drawLine(1,5);
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  2. #2
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    Default Re: drawing on a QPixmap or QImage

    Pass a pointer to the pixmap.
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  3. #3
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    Default Re: drawing on a QPixmap or QImage

    i.e.
    Qt Code:
    1. ...
    2. QPainter pixPaint(&pixmap);
    3. ...
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    Qt Assistant -- rocks!
    please, use tags [CODE] & [/CODE].

  4. #4
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    Default Re: drawing on a QPixmap or QImage

    Try adding a brush like
    Qt Code:
    1. QPainter pixPaint(&pixmap);
    2. QBrush brush2( Qt::blue);
    3. pixPaint.setBrush(brush2);
    4. pixPaint.drawLine(1,5);
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    Hope that helps!
    Last edited by qtUser500; 26th February 2009 at 16:50.

  5. #5
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    Default Re: drawing on a QPixmap or QImage

    Quote Originally Posted by spirit View Post
    i.e.
    Qt Code:
    1. ...
    2. QPainter pixPaint(&pixmap);
    3. ...
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    Thanks for all your inputs.

    I tried added a brush/ pen to QPainter, but I still don't see the line.

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    Default Re: drawing on a QPixmap or QImage

    What is the current code?
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  7. #7
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    Default Re: drawing on a QPixmap or QImage

    I tried the code

    Qt Code:
    1. QPainter pixPaint(&pixmap);
    2. QPen myPen( Qt::green);
    3. pixPaint.setPen(myPen);
    4. pixPaint.drawLine(5,5, 10,10);
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    Default Re: drawing on a QPixmap or QImage

    This seems to work quite fine for me:
    Qt Code:
    1. #include <QApplication>
    2. #include <QPixmap>
    3. #include <QPainter>
    4. #include <QLabel>
    5.  
    6. int main(int argc, char **argv){
    7. QApplication app(argc, argv);
    8. QPixmap px;
    9. px.load(argv[1]);
    10. QPainter p(&px);
    11. p.setPen(Qt::blue);
    12. p.drawLine(5,5, 40, 40);
    13. p.end();
    14. QLabel label;
    15. label.setPixmap(px);
    16. label.show();
    17. return app.exec();
    18. }
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  9. #9
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    Default Re: drawing on a QPixmap or QImage

    I am not able to compile, get an error C2512: 'QLabel' : no appropriate default constructor available. Tried using
    Qt Code:
    1. QLabel *label = new QLabel(QWidget*); // but this needs a QWidget
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    1. Does a pixmap always need a QLabel, for it to be set to?
    2. I tried on a different widget like canvas and listbox, here I see the pixmap, but not the line drawn over the pixmap.
    Last edited by gren15; 27th February 2009 at 18:57.

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    Default Re: drawing on a QPixmap or QImage

    I am not able to compile, get an error C2512: 'QLabel' : no appropriate default constructor available.
    Did you compile the code wysota gave ?? if changed, what did you write ?
    1. Does a pixmap always need a QLabel to be set to?
    Nope, pixmap is a paintdevice, and can be rendered onto any widget. using QLabel for showing pixmap is the simplest way since you dont need to DRAW the pixmap onto the painter, but simply set a pixmap for the label.
    2. I tried on a canvas, where I can get to see the pixmap, but not the line drawn over the pixmap.
    Can we see what you tried ?

  11. The following user says thank you to aamer4yu for this useful post:

    gren15 (3rd March 2009)

  12. #11
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    Default Re: drawing on a QPixmap or QImage

    The eventual direction I want to take is to create a listbox with pictures of animals. I want to select a few pictures (Eg grassland animals) and surround them with a green rectangle.

    This is what I tried:

    Qt Code:
    1. QPixmap pxWolf ("wolf.jpg");
    2. QPainter *animalPainter = new QPainter(&pxWolf );
    3.  
    4. QListBoxPixmap *animalItem = new QListBoxPixmap(animalListBox, pxWolf, "wolf"); // displays fine, I see the pixmap and the text
    5.  
    6. // I want to border this animal with a green line
    7. QPainter *borderPainter = new QPainter(animalItem);
    8. borderPainter->setPen(Qt::green);
    9. borderPainter->begin(&pxWolf);
    10. borderPainter->drawRect(QRect(1,1, 10,10)); // I don't get to see this green rectangle
    11. borderPainter->end();
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    I will be glad for any help on this. Thanks

  13. #12
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    Default Re: drawing on a QPixmap or QImage

    Is the question related to Qt3?
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  14. #13
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    Default Re: drawing on a QPixmap or QImage

    Opps! Forgot to mention it .. yes it is using Qt3.

  15. #14
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    Default Re: drawing on a QPixmap or QImage

    Can you prepare a minimal compilable example reproducing the problem?
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  16. #15
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    Default Re: drawing on a QPixmap or QImage

    Tweaking the QT listbox example, I have enclosed an executable version of what I want to do.

    Quick question: To resize the pixmap to a different size from the default size, I tried the resize function but it cuts the image, not resize it. I have also added this in the enclosed file.

    Qt Code:
    1. pixmap.resize(50,80);
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    Attached Files Attached Files
    Last edited by gren15; 2nd March 2009 at 17:34.

  17. #16
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    Default Re: drawing on a QPixmap or QImage

    Thanks everyone. I was able to resolve it.
    Last edited by gren15; 3rd March 2009 at 20:03.

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